1. Field of the Invention
The invention relates generally to a hydro-mechanical transmission particularly suited for use in an agricultural tractor. More specifically, the invention relates to an infinitely-variable hydro-mechanical transmission and a valve assembly for controlling the hydro-mechanical transmission.
2. Related Technology
Hydro-mechanical transmissions are transmissions that combine a mechanical transmission with a hydrostatic unit. Mechanical transmissions generally have simple and reliable designs with relatively high performance efficiency (i.e. low power loss). However, purely mechanical transmissions are not infinitely variable and are thus restricted to particular speeds. In many applications, particularly agricultural tractors, it is desirable for the transmission of the vehicle to be infinitely adjustable. Hydrostatic transmissions typically are infinitely variable. However, hydrostatic transmissions are less efficient than mechanical transmissions.
In order to satisfy space limitations, reduce cost, increase efficiency and provide an infinitely variable speed, hydro-mechanical transmissions have been developed to combine the advantages of both types of transmissions. Hydro-mechanical transmissions are typically of a split power input type, where a hydrostatic unit and a mechanical transmission are both driven in parallel by the vehicle's engine. The hydrostatic output power is combined with the split mechanical power input from the engine to produce hydro-mechanical output power having a variable speed within each of a plurality of gear sets. More specifically, the mechanical transmission includes a plurality of gear sets that are selectively engaged and, within a given gear set, the hydrostatic output power is infinitely variable to vary the output speed.
The hydrostatic unit is a closed hydraulic system having a pump that converts rotational motion from the engine input shaft into fluid flow and a motor (hydraulically driven by the pump) that converts the fluid flow back into rotational motion that is delivered to the mechanical transmission. The pump is a variable-displacement pump so that the output speed of the motor is infinitely variable. More specifically, the variable-displacement pump includes an adjustable mechanism, such as a swash plate, that is controlled by a valve assembly. The valve assembly controls the angle of the swash plate to adjust the fluid flow within the hydrostatic unit thereby adjusting the hydrostatic output power.
Current hydrostatic units include microcontrollers and other electronic devices for controlling the angular position of the swash plate and to shift between gear ranges. However, these design features may increase the part cost, maintenance costs, and overall complexity of the hydrostatic unit. Furthermore, repair and maintenance of these hydrostatic units may require equipment, such as diagnostic machines, that is expensive and not readily available in some geographic areas.
Additionally, current hydrostatic units are controlled by several levers and controls, such as one or more controls for adjusting the operative gear set and one or more levers for controlling the speed and torque within the operative gear set. The multiple controls and levers may be difficult or inconvenient for use by the tractor operator.
It is therefore desirous to provide a hydro-mechanical transmission having a simple and reliable valve assembly for shifting between operative gear sets and for controlling output speed within each gear set and to provide simple and convenient controls for the operator of the vehicle.